The present invention relates to a quadrature modulation circuit and, more particular to a quadrature modulation circuit applicable to a radio communication apparatus having an analog and digital transmission and reception capability.
A conventional quadrature modulation circuit of the type described at least includes a modulator for analog modulation, a modulator for digital modulation, and a switch for selecting either of the outputs of the two modulators. The problem is, therefore, that the two modulators have to be accompanied by various extra circuit parts including local oscillators, complicating the construction of the circuit. Further, the switch has to be switched at high frequency in selecting either of the outputs of the analog and digital modulators. To cope with noise particular to high frquency switching, the modulation circuit has to be provided with a special configuration.